The efference copy account of the tickle effect (i.e., our inability to tickle ourselves) predicts no tickle effect (i.e., an ability to tickle ourselves) when the trajectory of a tactile stimulus is perturbed relative to the associated movement, and there is evidence in support of this. The active inference account, however, predicts the tickle effect should survive trajectory perturbation. We test these accounts of the tickle effect under the hypothesis that previous findings are due to attentional modulation, and that the tickle effect will be found in a paradigm with no conscious attention directed to the trajectory perturbation. We thus expected to find support for active inference. Our first experiment confirms this hypothesis, while our second seeks to explain previous findings in terms of the modulation of the tickle sensation when there is awareness of, and different degrees of attention to, the spatial tactile and kinesthetic trajectories.
The effect of the body transfer illusion on the perceived strength of self- and externally-generated "tickle" sensations was investigated. As expected, externally generated movement produced significantly higher ratings of tickliness than those associated with self-generated movements. Surprisingly, the body transfer illusion had no influence on the ratings of tickliness, suggesting that highly surprising, and therefore hard to predict, experiences of body image and first-person perspective do not abolish the attenuation of tickle sensations. In addition, evidence was found that a version of the rubber hand illusion exists within the body transfer illusion. We situate our findings within the larger debate over sensory attenuation: (1) there is an attenuation of prediction errors that depends upon the context in which sensory input is predicted (i.e., efference copy), and (2) sensory attenuation is a necessary consequence of self-generated movement irrespective of context (i.e., active inference). The results support the notion of active inference.